1. Field of the Invention
The present invention relates to optical communication systems, and more particularly to an apparatus and method for tuning an optical interferometer.
2. Description of the Prior Art
An optical interferometer provides a means to detect changes in the wavelength of light by causing a change in light level resulting from the change in the interference state in its output. Optical interferometers made using optical fiber or silica waveguide are not stable devices. They are particularly susceptible to uncontrollable conditions, such as temperature variations. As the temperature Ad proximate the optical interferometer changes, the path length of the optical fibers or silica waveguide making up its legs likewise change. This results in a change in the interference pattern created by the optical interferometer. To compensate, the optical interferometer must be tuned continuously.
An apparatus and method for tuning an optical interferometer is known in the art. An example of such an apparatus is described in an article by Eric A. Swanson, Jeffrey C. Livas and Roy S. Bondurant, entitled xe2x80x9cHigh Sensitivity Optically Preamplified Direct Detection DPSK Receiver With Active Delay-Line Stabilization,xe2x80x9d in IEEE Photonics Technology Letters, Vol. 6, No. 2, Feb. 1994. This article describes an optical communication system that modulates digital information onto transmitted light using differential phase shift keying (DPSK) and then demodulates this information using an actively tuned unbalanced Mach-Zehnder optical interferometer that is tuned using an apparatus and a method known in the art. The unbalanced Mach-Zehnder optical interferometer has an additional optical path length in one leg that provides a propagation delay duration of one data bit. The imbalance in the Mach-Zehnder optical interferometer enables light in one data bit to be optically interfered with light in the data bit immediately following this data bit. The relative state of optical phase between these two DPSK data bits determines in which of the two output legs of the interferometer light is produced provided that the unbalanced Mach-Zehnder optical interferometer is properly tuned within a fraction of a wavelength of the light. Light produced from one leg constitutes digital xe2x80x9conesxe2x80x9d while light produced in the other leg constitutes digital xe2x80x9czerosxe2x80x9d in the transmitted digital information signal. This article also describes an apparatus and a method for using optical amplification to improve receiver sensitivity that utilizes a doped optical fiber amplifier to boost the signal level and a Fabry-Perot narrow band filter to remove the out-of-band amplified spontaneous emission (ASE) introduced by the fiber amplifier.
The apparatus described in the article includes a laser and a phase modulator for producing an optical DPSK signal at a preselected wavelength, a 10 GHz tunable fiber Fabry-Perot filter and an automatic controller for dithering the pass band wavelength of the filter so as to keep the peak of the filter at the optical signal wavelength, a tunable unbalanced Mach-Zehnder optical interferometer, a dual balanced detector and a feedback electronic circuit coupling the signal developed across one detector of the balanced detector to one leg of the Mach-Zehnder interferometer. Two different approaches are described for tuning the optical path length in the unbalanced Mach-Zehnder optical interferometer. In the first approach, the interferometer is made of optical fiber and one leg of the interferometer is wrapped around a piezoelectric transducer (PZT) that enables an electronic signal to stretch the fiber, thereby increasing the optical path length. In the second approach, the interferometer comprises a silica integrated optical waveguide with an integral thermal heater that enables an electronic signal to increase the temperature of one leg of the interferometer, thereby increasing the optical path length. To tune the Mach-Zehnder interferometer a small electronic dither signal is applied to the actively tuned optical path length to provide a feedback signal for the electronic controller. This enables proper adjustment of the optical path length. Electronic synchronous detection techniques on this dither signal are used to provide the appropriate corrections to the optical path length, enabling the error in tuning to be below an acceptable level.
The prior art approaches for actively tuning an optical interferometer have several disadvantages. First, they introduce an undesired optical intensity dither on top of the original optical communication signal that is intended to be extracted. This dither arising from the intentional dither of the optical path length is actually a source of noise that degrades the fidelity of the original communications signal. Second, the approach using the heater to perform the dither and tuning is restricted to relatively low frequencies of dither due to the relatively large thermal time constant of the heater. Third, the approaches introduce a small dithering variation in the interference state delivered at the output of the Mach-Zehnder interferometer. This precludes the use of the interferometer in applications where an absolute quiet state of interference must be maintained.
What is needed, therefore, is an apparatus for stabilizing an optical interferometer that utilizes an additional optical signal to tune the optical interferometer without introducing any dither in its optical path length.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides generally, in a first aspect, an apparatus and a method for using an additional optical signal having a wavelength that differs by a factor of two from the wavelength of the light originally detected by an optical interferometer to actively tune and stabilize the optical interferometer.
Briefly, the apparatus comprises a dithering signal generator, an optical interferometer having an optical path length that is tunable, means for generating an additional optical signal having a wavelength that differs from the original wavelength on which the optical interferometer is to act by a multiple of two, and means for applying the dithering signal to the first optical signal so as to slightly vary the wavelength about the multiple of two. The optical interferometer is responsive to the original optical signal to develop a first interference pattern when the interferometer is properly tuned. In this tuned state, the optical interferometer responds to the dithered optical signal to develop a similar second interference pattern. When the optical path length of one leg in the interferometer is changed, detector means responsive to the second interference pattern develop an electronic feedback signal indicating that the first interference pattern is not present, and that the interferometer requires an optical path length adjustment to become tuned. A feedback loop responds to the dithering signal and the electronic feedback signal to produce the optical path length adjustment drive signal. The optical path length adjustment drive signal serves to tune the optical path length until it reaches the prescribed value, thereby producing the first interference pattern and stabilizing the optical interferometer.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures to below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description.